Filter for suppression of harmonics



Patented June 8, 1943 one AES

. 2,321,375 FILTER FOR SUPPRESSION or HARMONICS James L. Finch, EastRockaway, N.

Y., as'signor v to Radio Corporation of America, a corporation ofDelaware Application May 31, 1941, Serial No. 396,097

(onus-44) 8 Claims.

My invention relates to high frequency filters for suppressingundesirable harmonics of a fundamental output frequency. Such harmonicshave in the past caused considerable interference with other services.In many cases no effective means have been used for their suppressionand in other cases they have only been partially suppressed, or veryexpensive equipment has been required for their suppression.

It is an object of my invention to provide a radio frequency filter foruse in a transmitter wherein the presence of harmonic components in theoutput of such a transmitter is substantially eliminated.

It is another object of my invention to provide a simple filterstructure the elements of which are inexpensive and easily adjusted.

Other objects and advantages of my invention will be made apparent inthe description to follow. This description is accompanied by a drawingin which Figure 1 shows a circuit diagram of a preferred embodimentofthe invention; and

Fig. 2 is a table giving the impedance values of certain of the elementsof the filter system as shown in Fig. 1 when adjusted to optimumoperating conditions.

Referring to Fig. l, I show a transformer I! connected to receive theoutput of the last stage of a transmitter. The load l8 may be of anydesired type such as an antenna. consists of conductors 2| and 22. I8both have neutral points which are connected to ground. Parts I, 2, 3, 4and 5 are variable or adjustable condensers, while parts ll, l2, I3, I4, I5 and I6 are variable or adjustable inductors. These parts areconnected in the circuit as shown and their values are so chosen for anyparticular fundamental operating frequency as to pass the fundamentalfrequency with very little attenuation but to have a very appreciableattenuating effect at all harmonic frequencies. "The input side of thisfilter, considered as a unit, is connected to the secondary oftransformer I] through conductors l9 and 20.

Referring now to Fig. 2, I show in the table thereof certain impedancevalues of the several reactances indicated by reference numbers inFig. 1. These reactances are correlated with respect to a givenfundamental frequency. In the Parts I? and last column of this table theattenuation in decibels is given for the fundamental and each of theharmonics up to the seventh. The table itself is based upon actualperformance records of a Its feed line certain embodiment whichIconstructed and used in a-transmitter having a push-pull fundamentalfreqiiency output. This table also gives impedances of combinations ofthese parts and the attenuation which is theoretically possible if weassume that the load impedance is 300 ohms each side of neutral for allof the frequencies I under consideration and if we assume the Q'of theinductors is 500 and of the condensers is infinity. One column gives thetype of currents which predominate at a particular'frequency. Asindicated in this column, the currents at the fundamental frequency andat odd harmonics of the fundamental are of the type known as push: pull.This means that at a particular instant when the current in conductor I9is flowing in one direction the current in conductor 20 is in theopposite direction. At all even harmonic frequencies the type of currentis push-push. This means that at a particular instant when the currentin conductor I9 is flowing in one direction it is also flowing in thissame direction in conductor 20. The return path for these currents isthrough the connections shown dotted, which include paths through theearth.

It should be noted that the values of the parts are so chosen that atthe fundamental frequency pi type low pass filter,

these circuits act as a the cut-off frequency being set for a frequencyhigher than the fundamental frequency but lower than the second harmonicfrequency. It should further be noted that at the second harmonicfrequency reactances 3, I3, 4, I4 and I5 form a tuned short circuit toground from conductors 2! and 22. This greatly attenuates the secondharmonic otherwise flow to the antenna.

harmonic frequencies reactances 3, form a tuned short circuit betweenconductors 2| and 22. At the fourth harmonic parts I, 2'and 5, [6 form atuned short circuit between conductors l9 .and 20 and ground. At thefifth and seventh harmonics and at all higher odd har- At the thirdmonics these reactances effectively form an L currents which would 13,4and 14' tion between said ca pacitors, .a second -shunt circuit acrosssaid output inductive, capacitive; capacitive and inductive the orderstated, and -an tion between the two said capacitive elements, a

elements arranged in inductive connection from ground to the junc sourceof input. energy for saidfilter having a grounded center-tap, and aloadconnected across said output terminals also havinga center tap.

2. The combination according to claim 1 wherein the reactance valuesassigned to the several elements of the filter are such as to produce anattenuation of the fundamental frequency by not more than .01 db. and anattenuation of the second to seventh harmonics by not less than 35 3;The combination according to claim 1 wherein the reactance valuesassigned to the several elements of the filter are such as to produce angrounded terminals comprising 3 elements of the filter are such as toproduce ah attenuation of the fourth harmonic by not less than 68 db.

6. The combination according to claim 1 wherein the reactance valuesassigned to the several elements of the filter are such as to produce anattenuation of the fifth. sixth and seventh harmonies by as much as 35db., 4'1 db. and 37 db. respectively.

7. A two-conductor filter for passing a fundamental'frequency withminimum attenuation and for attenuating harmonics of said fundamentalfrequency by at least 35 decibels, said filter comprising equivalentinductances connecte'd in each of the leads from the two input terminalsto the two output terminals, shunting capacitors connected across theinput terminals, shunting reattenuation of the second harmonic by notless than 73' db.

4. The combination according to claim 1 wherein the reactance valuesassigned to the several elements of the filter are such as to produce anattenuation of the third harmonic by not less than 120 db.

5. The combination according to claim 1 wherein thereactance valuesassigned ot the several actances symmetrically disposed across theoutput terminals, and means for dissipating to ground certain of saidharmonics, the last said means comprising a parallel tuned circuitconnected between groundand the junction between said shuntingcapacitors, and an inductance connected between ground and a center tapon the circuit of said shunting reactances.

8. A low pass filter suitable for push-pull transmission of a givenfundamental frequency, and having high attenuation properties withrespect to certain harmonics of said frequency, said filter having aninductance in each of its two series conductors, reactancessymmetrically arranged in each of two shunt circuits, the first of whichis connected across the input terminals and the second across the outputterminals, and a parallel resonant circuit connected to ground from amid-tap on said first shunt circuit whereby the fourth harmoniccomponent is dissipated to ground.

JAMES L. FINCH.

